1. Field of the Invention
The present invention relates to an electro-optic device and to a method of operating an electro-optic device.
2. Description of Related Art
Crystals that lack a centre of symmetry can show a linear electro-optic (EO) effect where the induced birefringence is linearly proportional to the applied electric field. The polarisation components of an optical beam propagating in such crystals have different phase velocities and thus produce a phase shift that depends on the intensity and orientation of the applied field. An application of the EO effect is in optical modulators, in which a signal-controlled EO crystal is used to modulate an optical beam.
Semiconducting high-resistivity crystals are attractive EO materials. High resistivity enables the crystal to sustain electric biasing fields with negligible power consumption, which represent a preferred condition for EO modulation. In this context, with high-resistivity semiconductors it is meant semiconducting materials with resistivity comprised between 107 and 1012 Ωcm.
Among high-resistivity semiconducting materials, crystals with zinc-blende (cubic) crystalline structure have been widely studied. Because of their optical isotropy in the absence of an externally applied electric field, there is no need to apply a compensating biasing voltage or to add compensating birefringent waveplates on the optical path to achieve polarisation independence. Examples of zinc-blende EO crystals are GaAs, CdTe, ZnS-cubic, CdZnTe or ZnTe.
Another important class of high-resistivity semiconductors includes the crystals having hexagonal (wurtzite) structure, such as CdS, ZnS-wurtzite, CdSe or CdSSe.
CdTe and CdZnTe are well known as substrate materials for high-energy or infrared detectors.
CdTe-based radiation detectors may exhibit a poor stability due to a charge, or local, polarisation effect. Local polarisation, which is induced by applying a bias voltage to a detector or by irradiating with high photon fluence on a detector, causes a progressive decrease of counting rate and pulse height with time. Sato T. et al. in “Local polarization phenomena in In-doped CdTe x-ray detector array”, IEEE Transactions on Nuclear Science, vol. 42, p. 1513 (1993), study the local polarisation phenomena in CdTe:In detector arrays. A temperature dependence of charge polarisation was observed and, as the detector temperature increased, the local polarisation phenomena tended to diminish. This result was said to indicate that the local polarisation phenomena resulted from ionisation of deep levels that induced the space charge. Since the raise of the detector temperature releases the trapped charges easily from the deep level, the polarisation phenomena tend to diminish with the rise of the detector temperature. U.S. Pat. No. 5,248,885 discloses a x-ray radiation detector formed with a compound semiconductor, such as CdTe or GaAs. During radiation detection, the detector can be heated at a suitable temperature or irradiated with infrared rays having an energy smaller than the bandgap of the semiconductor compound and larger than the energy of the trap level present in said compound in order to compensate for a reduction in the output of the detecting element occurring with a high-dose incidence.
Electro-optic modulators in lithium niobate (LiNbO3), an insulating ferroelectric oxide material, are widely used in digital communication systems. Some optical applications using LiNbO3 crystals are known to be hampered by the so-called optical damage, which leads to a localised light-induced change of the refractive index. For LiNbO3, optical damage is mainly caused by the photorefractive effect. The damage process due to light exposure can occur gradually over days or hours, or, in case of high optical powers and short wavelengths, in seconds.
Japanese patent application No. 11-174390 describes a LiNbO3-based optical modulator, which is heated at a temperature above room temperature to reduce the optical damage induced by a short-wavelength (e.g., 442 nm) light beam.
U.S. Pat. No. 3,446,966 discloses that a LiNbO3-based optical modulator exhibits a significant EO effect when maintained at a temperature of at least 50° C.
The study of CdTe and, more recently, of CdZnTe for infrared light modulators has received ample interest in the past years. In U.S. Pat. No. 3,955,880, cadmium telluride is said to be a suitable material for infrared radiation modulators because of its high resistivity and of the limited absorption rate for infrared radiation. A CdTe-based module for optically activated switching of optical beams is described in “Opto-optical switching in the infrared using CdTe”, by Steier W. H. et al., Optics Letters, vol. 14, p.224 (1989).
The EO effect is generally expressed by the EO or Pockels coefficients rij of a third-rank tensor, where the first subscript refers to a tensor component of the index ellipsoid and the second to the electric field vector. The EO coefficients are characteristic parameters of each EO material and relate the applied electric field to the variation of birefringence induced by that electric field. For example, zinc-blende crystals belong to the {overscore (4 )}3 m symmetry group that has three equal Pockels coefficients, i.e., r41=r52=r63.
Very often, an EO material is characterised by means of the figure of merit. For a zinc-blende crystalline structure, the figure of merit is equal to the product of the electro-optic factor r41 and the third power of the refraction index n0 for the ordinary ray, namely r41n03. For an hexagonal crystalline structure of the 6 mm group symmetry, such as that of CdS, the figure of merit is given by r13−(n0/ne)3r33, where ne is the refractive index for the extraordinary ray.
A light beam transmitted across an EO crystal alters its state of polarisation when voltage is applied to the crystal by causing a phase retardation between orthogonal polarisation components of the beam. The relative phase retardation, Γ, is given by the following expression,
                              Γ          =                                                    2                ⁢                                                                  ⁢                π                            λ                        ⁢            Δ            ⁢                                                  ⁢            nL                          ,                            (        1        )            where Δn is the electrically induced birefringence (the difference in refractive index for the two polarisations of light), which is proportional to the applied electric field, L is the crystal length and λ the wavelength of light that is being used.
Free-charge carriers, photoexcited by the unavoidable absorption of light from the incoming beam, result in a volume space-charge distribution that locally shields the externally applied field: This mechanism lowers the Pockels effect, which can be partially or completely inhibited at certain optical intensities. A possible explanation of this phenomenon is the formation of space-charge regions in the bulk of the material, due to carrier photogeneration from intra-gap levels.
Milani et al. in “Characterization of electro-optic shielding effect in bulk CdTe:In crystals”, Journal of Crystal Growth, vol. 214/215, p. 913 (2000), describe measurements on CdTe:In. The EO actual figure of merit was observed to decrease with the decrease of the modulation frequency for frequencies below a certain threshold value, or cut-off frequency, that depended on the incident optical power. Measurements were carried out for samples at 300 K (room temperature) and at 80 K. The cut-off frequency was seen to lower by a decade in passing from 300 K to 80 K. The authors concluded that loss in EO yield could be minimised by a suitable reduction in operating temperature and sample dimensions, having defined the optical power of the signal to be processed.
In “High-pressure Bridgman grown CdZnTe for electro-optic applications” by Zappettini et al., Journal of Electronic Materials, vol. 30, p. 743 (2001), hereafter referred to as Zappettini's article, the decrease of the figure of merit with the increase of optical beam intensity is reported for a Cd0.9Zn0.1Te sample; A decrease of the figure of merit with the increase of beam power was observed for a modulation frequency of 1 Hz, reaching a 25% reduction at an incident optical power Pin=1 mW. Measurements on the Cd0.9Zn0.1Te crystal are compared to measurements on a CdTe:In sample. For optical beams at a wavelength of 1530 nm and frequencies of less than 100 Hz, the value of figure of merit for CdTe:In decreases, whereas the performance of Cd0.9Zn0.1Te does not substantially change at low frequencies. CdZnTe was used as basic material to implement a 2×2 EO switch operating on free-space propagating signals.
WO 01/90811 describes more in detail a CdZnTe-based EO switch. The EO switch, for infrared light modulation, includes CdxZn1-xTe, where the Cd molar fraction x is between 0.7 and 0.99.
Applicants have observed a strong reduction at low frequencies of the figure of merit of high-resistivity semiconductors when the wavelength of the optical beam that passes through the crystal is of 1300 nm or smaller, even if the incident optical power is low, for instance of 0.1 mW. Applicants have found that this may lead to degradation in optical modulation at low-frequency EO modulation or switching, e.g., in quasi-static switches for use in optical cross-connects.